This invention has for a specific objective the production of gradients in the refractive index of glass articles through the use of ion exchange techniques, such technology being applicable to the optical engineering field, particularly the making of high performance fiber optic components.
The current interest in making high performance fiber optic components has led to a resurgence of efforts to further pursue optical engineering technologies. One such technology being developed combines ion exchange techniques with photolithography for integrating optical waveguides in a glass substrate. Optical waveguides can be embedded in a glass substrate to create a wide variety of optical circuits and passive optical functions for devices such as splitters, stars, wavelength division multiplexers, and optical power taps. These functions are readily integrated into a single glass substrate to facilitate component miniaturization and controlled performance.
Such research was presented by Kaps, Karthe, Muller, Possner, and Schreiler in "Glasses for Optoelectronics," ECO Proceedings, Paris, France, Vol. 1128, Apr. 24-27, 1989, wherein a special glass type that is favorable for silver-sodium ion exchange is described. This special glass is used for fabricating channel waveguides and waveguide devices whereby the glass substrate is covered with a metal film. Patterns are generated by photolithography and a wet etching technique using electron beam written masks.
Ion exchange, a technique for producing gradients in the refractive index of glass articles, has been in use since the early sixties. The essence of this method lies in the exchange of ions having different polarizabilities, viz., exchanging one alkali ion for another. For example, U.S. Pat. Nos. 3,524,737 and 3,615,322 describe glass strengthening techniques whereby the sodium ion in glass is replaced by potassium and copper ions, respectively. Similarly, U.S. Pat. No. 3,615,323 describes a similar glass strengthening technique, yet with the sodium ion being replaced by a lithium ion. Modest changes in refractive index are achieved by such exchanges.
Thallium has commonly been chosen over other elements as a doping ion to create regions with a higher refractive index. Large changes in the refractive index of glasses have been achieved by the ion exchange of thallium; however, the use of thallium is limited to some extent by its toxicity. Nevertheless, thallium is the ion most often used today in ion exchange processes in spite of its inherent toxicity problems.
The instant invention discloses a promising alternative to thallium in creating large gradients in the refractive index of silica-based glasses--the exchange of Ag.sup.+. The exchange of silver for an alkali metal produces a change in refractive index comparable to that produced by the thallium exchange, yet without the inherent toxicity problems. The potential advantages of this exchange have not been previously realized because the introduction of more than minimal amounts of silver into a silicate glass by ion-exchange techniques has invariably led to extensive chemical reduction of silver, with attendant increase in attenuation in the optical path. Hence, the intense color which characterizes the formation of colloids when silver is reduced is unacceptable for optical waveguide applications and, indeed, for most optical applications where an essentially colorless, transparent glass is required.
From the studies on silver dissolved in borosilicate glass, particularly in the photochromic glasses, it can be deduced that reduction of silver ions can result from the extraction of electrons intrinsic to the glass network. Furthermore, the relative ease with which this extraction occurs varies strongly with the composition of the glass. Indeed, glasses were found in which no reduction occurred.
Polyvalent impurities such as arsenic or tin can, of course, provide electrons leading to the reduction of a small amount of silver and consequently can cause a small degree of coloration. It is unrealistic, however, to believe that the small levels of impurities typically found in these glasses can be responsible for the considerable coloration observed in alkali silicate glasses upon the introduction of modest amounts of silver. Moreover, scrupulous efforts to exclude polyvalent ions from the glass failed to prevent extensive silver reduction. It is therefore an object of this invention to provide a prescription for making glasses in which the physical properties can be varied within moderately wide limits, but in which the amount of silver reduction does not exceed that caused by polyvalent ion impurities.